Energy dependent XPS measurements on thin films of a poly(vinyl methyl ether)/polystyrene blend concentration profile on a nanometer resolution to understand the behavior of nanofilms.

Marcel Gawek,Andreas Schönhals,Sherif Madkour,Paulina Szymoniak,Jörg Radnik

doi:10.1039/d1sm00656h

Marcel Gawek, Andreas Schönhals + Show 3 more

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https://doi.org/10.1039/d1sm00656h

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Abstract

The composition of the surface layer in dependence from the distance of the polymer/air interface in thin films with thicknesses below 100 nm of miscible polymer blends in a spatial region of a few nanometers is not investigated completely. Here, thin films of the blend poly(vinyl methyl ether) (PVME)/polystyrene (PS) with a composition of 25/75 wt% are investigated by Energy Resolved X-ray Photoelectron Spectroscopy (ER-XPS) at a synchrotron storage ring using excitation energies lower than 1 keV. By changing the energy of the photons the information depth is varied in the range from ca. 1 nm to 10 nm. Therefore, the PVME concentration could be estimated in dependence from the distance of the polymer/air interface for film thicknesses below 100 nm. Firstly, as expected for increasing information depth the PVME concentration decreases. Secondly, it was found that the PVME concentration at the surface has a complicated dependence on the film thickness. It increases with decreasing film thickness until 30 nm where a maximum is reached. For smaller film thicknesses the PVME concentration decreases. A simplified layer model is used to calculate the effective PVME concentration in the different spatial regions of the surface layer.

Highlights

Polymer blends have been an integral part of materials science due to their numerous applications
Such adventitious carbon is obtained on most air-exposed samples and has nothing to do with the employed polymers or its purity
We have chosen an experimental approach and compared the composition of a pure poly(vinyl methyl ether) (PVME) sample with a thickness of 15 nm which was kept in a vacuum, from preparation until the measurement at the theoretical IC–O/IC–H ratio of 2. (A C1s X-ray Photoelectron Spectroscopy (XPS) spectrum of PVME is given in the Electronic supplementary information (ESI),† see Fig. S1.) By handling in vacuo, the surface contamination can be minimized

Summary

Introduction

Polymer blends have been an integral part of materials science due to their numerous applications. Angle-dependent (conventional) XPS could be used to estimate the composition of a surface layer of a sample.[36,47] In this work, an alternative approach is introduced to study the compositional heterogeneities in thin films of polymer blends as a function of their film thickness, where a range of different photon energies is considered. This approach leads to different information depth and to detailed information about the compositional heterogeneities in the respective ranges without sample destruction.

Experimental

Results and discussion

Conclusions